If you have a rare disease, just hearing how much it costs to treat it could make you sicker.

The world’s priciest medicine, Soliris, costs an astonishing $409,500 a year for the average patient, Forbes.com said yesterday in its survey of the most expensive medicines on earth.

Most of the pricey pharmaceuticals target rare disorders that affect relatively few people.

Soliris, made by Alexion Pharmaceuticals, is given intravenously to treat paroxysmal nocturnal hemoglobinuria, or PNH, a rare disorder in which the immune system destroys red blood cells. About 8,000 Americans suffer from the disease, Forbes said.

But there is some good news.

Patients don’t have to dig that deep to pay for the drugs. Insurance covers most of the costs — and there is help with high co-payments or for those with no insurance.

Alexion spokesman Irving Adler said the high price of Soliris reflects several factors, “including an $800 million investment to develop the drug,” as well as a 15-year investment of time.

The second spot on Forbes’ list is held by Elaprase, which costs about $375,000 a year. The drug, made by Shire Pharmaceuticals, treats Hunter syndrome, an extremely rare metabolic disorder that affects about 500 Americans.